Gypsum wallboard is used in the construction of residential and commercial buildings to form interior walls and ceilings. Because it is relatively easy to install and requires minimal finishing, gypsum wallboard is preferred over plaster in almost all applications.
Gypsum wallboard consists of a hardened gypsum-containing core surfaced with paper or other fibrous material suitable to receive a coating such as paint. It is common to manufacture gypsum wallboard by placing an aqueous core slurry comprised predominantly of calcined gypsum between two sheets of paper thereby forming a sandwich structure. The aqueous gypsum core slurry is allowed to set or harden by rehydration of the calcined gypsum, usually followed by heat treatment in a dryer to remove excess water.
Conventionally in the manufacture of gypsum board, a pre-generated foam is added to the board core slurry mix to decrease the weight of the gypsum board. This foam is generated from a mixture of a liquid foaming agent, air and water in a suitable foam generating apparatus. The foamed gypsum slurry is then deposited upon a moving paper substrate, which, itself, is supported on a long moving belt. A second paper substrate is then applied on top of the slurry to constitute the second face of the gypsum board and the sandwich passes through a forming station, which determines the width and thickness of the gypsum board. In such a continuous operation the gypsum slurry begins to set immediately after passing through the forming station. When sufficient setting has occurred the board is cut into commercially acceptable lengths and then passed into a board dryer. Thereafter the board is trimmed, bundled, shipped, and stored prior to sale.
The majority of gypsum wallboard is sold in sheets that are four feet wide and eight feet long. The thickness of the sheets varies from about one-quarter inch to about one inch depending upon the particular grade and application, with a thickness of about one-half inch being most common. A variety of sheet sizes and thicknesses of gypsum wallboard are produced for various applications.
Gypsum wallboard is relatively dense (approximately 43.2 lbs/ft3), which equates to an overall weight of about 1,800 pounds per thousand square feet of one-half inch wallboard. Weight, rather than volume, determines the amount of gypsum wallboard that can be shipped by truck, and freight charges add significantly to the final cost of gypsum wallboard. Thus, it would be highly desirable to be able to reduce the overall weight of gypsum wallboard so that additional sheets could be shipped per truck. In addition, a lighter gypsum wallboard would be easier to handle and install at building sites.
As noted above, it is common practice to introduce air bubbles into the aqueous gypsum core slurry to create voids in the hardened core. The introduction of air bubbles reduces the density of the gypsum wallboard and also reduces the amount of water necessary to produce a workable core slurry. Air bubbles are typically formed via the addition of foaming agents during the formation of the slurry or by adding externally generated foam to the slurry. The externally generated foam is normally produced by incorporating air or other gas into an aqueous solution of a foaming agent, which is then mixed with the slurry. Commercial gypsum wallboard typically contains about 70% air (or void space) by volume. About 30% of that volume results from entrained air bubbles that are formed via the use of foaming agents.
As noted in Jobbins, U.S. Pat. No. 6,171,388, as the volume of air bubbles in gypsum wallboard increases, the strength of the wallboard tends to dramatically decrease, making the wallboard commercially unacceptable. Jobbins further notes at col. 1, lines 42-45, that “[e]xperience has taught that entrainment of air alone will not product a sufficiently improved lightweight wallboard that has adequate strength.” Generally speaking, a gypsum wallboard must be able to pass the objective testing criteria set forth in the ASTM C36 and C473-00 standards in order to be commercially acceptable.
Prior art attempts to reduce the weight of gypsum wallboard have involved dispersing lightweight particles (e.g., expanded vermiculite, perlite, or expanded thermoplastic polymeric resins) into the aqueous gypsum core slurry, or more recently, incorporating a latex emulsion and an excess amount of a nonionic surfactant or an acrylic polymer into the aqueous gypsum core slurry. Some of these methods are effective at reducing the overall weight of the gypsum wallboard, but are expensive and/or present other problems. Thus, prior art efforts to create lightweight gypsum wallboard have met with limited success.
In addition to freight costs discussed above, another significant factor adding to the final cost of gypsum wallboard is the energy required to remove excess water from the slurry during curing and drying. A certain amount of water is required to hydrate the dry calcined gypsum material, but in order to obtain a smooth, free-flowing, low viscosity mixture that can be formed into a sheet in a mold, it has been necessary to add two or three times as much water to the slurry as is needed for hydration. It would be highly desirable to reduce the amount of excess water that is necessary during the production of gypsum wallboard.